Conventionally, an aircraft is provided with an environmental control system in which air is taken from the compressor of one or more of its engines and is cooled and dried before being mixed with recirculated cabin air. The air mixture is then supplied to the aircraft cabin. Temperature adjustments are made to the air by mixing it with warm air taken from the engine or engines.
Such environmental control systems are effective in the provision of an acceptable aircraft cabin atmosphere. However, they do have drawbacks in that taking air from the engine's compressor has a detrimental effect upon overall engine operating efficiency and large amounts of ducting are required to direct the air around the aircraft.
A way of avoiding the drawbacks of such environmental control systems is disclosed in U.S. Pat. No. 4,546,939. In U.S. Pat. No. 4,546,939, an aircraft environmental control system is described which takes no air from the aircraft's engines. Instead, electrical power derived from the engine, or alternatively from a ground supply when the aircraft is parked, is used to drive a compressor which compresses ambient air. Air from the compressor is directed through a heat exchanger to cool it before entering a turbine through which it expands to provide further cooling and water vapour condensation. The cooled air is then passed through a further heat exchanger before being directed into the aircraft's cabin.
In the system of U.S. Pat. No. 4,546,939, the compressor and turbine are mechanically interconnected. This, unfortunately, has an adverse effect upon overall operating efficiency.
It is an object of the present invention to provide an environmental control system in which the above-mentioned drawbacks are substantially avoided.